Method of adjusting an adjustable piston-driver connection

ABSTRACT

A piston and a fastener driving element connection including interengaging threads on the piston and fastener driving element and a thermoplastic locking compound between the interengaging threads. The connection being adjustable by following a method including the steps of heating the threaded connection sufficient to soften the thermoplastic locking compound, effecting a relative turning movement between the fastener driving element and piston while the thermoplastic locking compound is softened to an extent sufficient to bring the fastener driving element and piston into the desired position of adjustment, and then allowing the softened thermoplastic locking compound to harden.

This is a division of application Ser. No. 07/321,756, filed Mar. 10,1989, now U.S. Pat. No. 5,000,365.

This invention relates to fastener driving devices and, moreparticularly, to the piston and fastener driving element assembly ofsuch devices.

The type of fastener driving device herein contemplated typicallyincludes a fastener driving element or driver which is mounted within adrive track within which successive fasteners are fed. The driver ismoved through repetitive cycles, each of which includes a drive strokeduring which the fastener is moved out of the drive track into theworkpiece and a return stroke. The fastener driving element is fixedlyconnected with a piston which is mounted within a cylinder for movementthrough a drive stroke and return stroke with the fastener drivingelement. The piston is driven by gases under pressure preferably in theform of compressed air, applied to an operative surface area of thepiston. While the invention is particularly suited to compressed airunits, there can be applicability to internal combustion and ballisticactuated units where relatively low operating temperatures arecontemplated.

The driver-piston assembly of a device of this type is inherentlysubjected to high stress forces tending to produce wear on the driverand disassembly of the driver from the piston. Due to the lattertendency, the usual connection between the fastener driving element andthe piston constitutes an interengaging thread connection. Thisconnection may take several forms. One form is to provide a centralthrough bore in the piston which is formed with internal threads and toprovide on the fastener driving element a cylindrical end portion theinner end of which terminates in a radially extending annular flange.The cylindrical end portion of the fastener driving element is providedwith exterior threads which mesh with the interior threads of thepiston. The usual arrangement is to effect a threaded interengagement ofthe two parts with the use of a locking compound, such as Loctite, withthe annular flange of the fastener driving element being torqued intoengagement with the piston at the portion thereof below the lower end ofthe bore. In arrangements of this type, the piston and fastener drivingelements are formed with flat surfaces so that in the final assemblyappropriate tools can be used to apply an appropriate torque to theassembly. The torquing of the intermeshing relationship between thethreads which creates substantial opposing axial forces in theintermeshing threads adds to the efficacy of the joint enabling thelength of meshing interengagement to be minimized. On the other hand,the need to provide the flange which permits a high torque to beapplied, materially increases the cost of the fastener driving element.

While torqued assemblies with flanged drivers are most prevalent, thereare units which are neither flanged nor torqued. An example of such aunit is included in a device identified as Hitachi Model NR83A. TheHitachi Model NR 83A includes a piston and fastener driving elementassembly in which the fastener driving element includes exterior threadswithout a torquing flange and meshing interengagement with the interiorthreads of the piston is accomplished without creating substantialopposing axial forces between the threads. A locking compound is used toset the final assembly. The arrangement does not contemplate convenientdisassembly since, with the elimination of the need to torque to theassembly, the usual tool engaging flat surfaces provided for the purposeof torquing have been eliminated.

Another type of assembly involves the extension of the cylindrical endof the fastener driving element to a point beyond the threaded bore andthen providing a locking nut on the extended end which serves to engagethe portion of the piston surrounding the upper end of the bore. With anarrangement of this type, it is possible to eliminate the use of thelocking compound if the nut is of the locking type, as, for, example, alock nut or the like. A disadvantage of the second type of connectionwherein a lock nut is utilized is that the arrangement tends to extendthe height of the tool and to add weight or increase the mass of thedriver-piston combination.

In situations where the tolls are used extensively, a driver can showsufficient wear as to require replacement for as many as three or moretimes during the life of the tool. The driver is a fairly expensive partwhich must be hardened by heat treatment or the like. The usual practiceis to replace the driver in the assembly by removing the same from thepiston and reconstructing the connection with a new driver. The wear onthe driver that requires replacing is wear on the lower fastenerengaging surface thereof. Where this wear gets to an extent equal toone-eighth of an inch, replacement is in order. Moreover, it will beunderstood that variation in the length of the driver to an extent ofone-eighth of an inch can result in a significant difference in theextent to which the head of the fastener is countersunk with respect tothe workpiece. It is also sometimes the practice to recondition worndrivers by grinding the lower end to provide a new fastener engagingsurface and to utilize shims when the reconditioned driver isreconnected with the piston. In all of the situations outlined above, itis necessary to effect an initial disconnection between the piston andthe old driver and then to effect a reconnection between the piston andeither a new driver or a reconditioned driver. In the case where finallocking is accomplished by a locking compound rather than a locking nut,new locking compound must be applied as the reconnection takes place.

An object of the present invention is to eliminate the need to effectreplacement of the driver or driver-piston combination when the driveris worn or requires readjustment to accommodate a flush head conditionrather than a countersunk condition by providing a method foreffectively adjusting the operating position of the fastener drivingelement with respect to the piston in the axial direction of the pistonwithout the necessity of disconnecting the interengaging threads andapplying new locking compound when the threaded reconnection isreestablished. In accordance with the principles of the presentinvention, this objective is obtained by utilizing a method comprisingthe steps of heating the threaded connection sufficient to soften thethermoplastic locking compound, effecting a relative turning movementbetween the fastener driving element and the piston while thethermoplastic locking compound is softened to an extent sufficient tobring the fastener driving element and the piston into the desiredposition of adjustment, and then allowing the softened thermoplasticlocking compound to harden.

Another object of the present invention is the provision of apiston-to-fastener driving element connection which is particularlysuited to the method of adjustment described above. This objective isobtained in accordance with the principles of the present invention byproviding a connection which includes interengaging threads on thepiston and the fastener driving element and a thermoplastic lockingcompound between the interengaging threads. The interengaging threadsinclude an interior thread having a pitch, a pitch diameter, and anaxial length defining an interior thread surface area and an exteriorthread having a pitch, a pitch diameter, and an axial length defining anexterior thread surface area. The difference in the pitch diameterbetween the interior and exterior threads is within a range of 0.000" to0.009" so as to define a clearance space between the interior andexterior thread surface areas to receive the thermoplastic lockingcompound when the threads are interengaged. The threads are interengagedin an initial position of operation wherein an initial portion of thelength of the interior thread is disposed in meshing relation with aninitial portion of the length of the exterior thread. The meshingrelation is devoid of significant opposing axial forces acting betweenthe threads. The interior and exterior thread surface areas within theinitial portions of the lengths thereof have the thermoplastic lockingcompound disposed in contact therewith and filling the clearance spacetherebetween in a hardened condition so as to secure the piston andfastener driving element together in the initial position of operation.The piston and fastener driving element have flat surfaces providedthereon for the purpose of enabling the same to be conveniently engagedby tools for effecting a relative turning movement therebetween intodifferent adjusted positions of operation. The piston and fastenerdriving element are movable from the initial position of operation inresponse to (1) the application of heat thereto sufficient to soften thethermoplastic locking compound and (2) a relative turning movementbetween the piston and the fastener driving element in either directioninto any desired adjusted position of operation while the thermoplasticlocking compound is in the softened condition and in one direction intoa final position of operation wherein a final portion of the length ofthe interior thread is disposed in meshing relation with a final portionof the length of the exterior thread so as to compensate for wear of thefastener driving element while in the initial position of operation andin subsequent adjusted positions of operation. The interior and exteriorthreads have portions of their lengths common to both the initialportions and the final portions defining a total thread surface areacontacted by the thermoplastic locking compound in any position ofoperation. The ratio of the total thread surface area to the operativepiston area is within the range of 0.540:1 to 0.373:1 so as to insurethat there is sufficient thread surface area contacted by the hardenedthermoplastic locking compound within the associated clearance space toeffectively secure the piston and the fastener driving element togetherin any position of operation.

Another object of the present invention is the provision of a piston tofastener driving element connection which is simple in construction,economical to manufacture, and effective in operation.

These and other objects of the present invention will become moreapparent during the course of the following detailed description andappended claims.

The invention may best be understood with reference to the accompanyingdrawings wherein an illustrative embodiment is shown.

IN THE DRAWINGS

FIG. 1 is a side elevational view of a fastener driving device showingthe piston and fastener driving element assembly embodying theprinciples of the present invention in dotted lines within the cylinderand drive track of the device;

FIG. 2 is an enlarged fragmentary vertical sectional view showing theconnection between the piston and the fastener driving element in aninitial position of operation;

FIG. 3 is a view similar to FIG. 2 showing the piston and fastenerdriving element connected in a final position of operation after themethod of adjusting in accordance with the principles of the presentinvention has been accomplished;

FIG. 4 is a schematic view illustrating various thread dimensionalrelationships pertinent to the present invention; and

FIG. 5 is a graph illustrating a range of dimensional characteristicsutilized in accordance with the principles of the present invention.

Referring now more particularly to the drawings, there is shown in FIG.1 a fastener driving device, generally indicated at 10. The fastenerdriving device 10 is illustrated merely to exemplify the type of deviceto which the present invention is applicable. The device 10, as shown,is of the pneumatically actuated type which includes a portable housing12 having a hollow handle 14 defining a reservoir connectable to asource of compressed air. The device includes a trigger actuated valvingmechanism, generally indicated at 16, for applying the compressed airwithin the reservoir onto a piston, generally indicated at 18, which ismounted for movement within a cylinder 20 through repetitive cycles ofoperation, each of which includes a drive stroke and a return stroke.The piston 18 is connected together with a fastener driving device ordriver, generally indicated at 22, which extends from the piston 18 andis mounted for movement with the piston through a drive track 24. In thetypical device 10 shown, a magazine assembly 26 is provided for feedingsuccessive fasteners into the drive track 24 to be driven by the fasterdriving element 22 outwardly thereof into the workpiece during the drivestroke of the piston 18. The return stroke can be effected in any knownfashion as, for example, a return plenum chamber arrangement (notshown).

It will be understood that the principles of the present invention haveapplicability to other types of fastener driving devices such asinternal combustion acutated devices and ballistic actuated devicesprovided their operating temperatures are fairly low.

Referring now more particularly to FIGS. 2 and 3, the piston 18 consistsessentially of a cylindrical body 28 made of aluminum alloy havingannular grooves 30 formed in the exterior periphery thereof forreceiving suitable O-ring seals (not shown). The cylindrical bodyincludes a top surface 32 defining with the O-ring seals the operatingpressure area of the piston. Extending through the cylindrical bodyalong the cylindrical axis thereof is a bore on which interior threads34 are formed. It will be noted that the central portion of the topsurface 32 is defined by an annular boss 36 within which the threads 34extend. The exterior of the boss is formed with a pair of opposed wrenchengaging flat surfaces 38.

The fastener driving element 22 is preferably made from bar stock steelwhich may be suitably heat treated. As shown, the bar stock steel isformed to include a cylindrical end portion 40 formed with exteriorthreads 42. It will be noted that the cylindrical end portion 40 of thefastener driving element 22 constitutes the largest transverse dimensionof the fastener driving element and that just below the cylindrical endportion 40 the fastener driving element is formed in two opposed wrenchengaging flat surfaces 44.

The piston 18 and fastener driving element 22 are connected together byeffecting a relative turning motion between the two as placing one ofthe two pairs of flat surfaces 38 and 44 in a vise and applying a wrenchor other suitable tool to the other.

Prior to the interengagement of the threads, they are suitably cleanedin accordance with conventional practice and then have applied to eachan amount of thermoplastic locking compound in accordance with knownpractices. Any suitable thermoplastic locking compound may be utilized.An exemplary embodiment is Loctite Grade 271, manufactured by LoctiteCorporation. The compound is applied to both the interior and exteriorthreads and a bead of compound can be observed being pushed ahead of thefastener driving element 22 as the fastener driving element 22 isscrewed into the piston 18. As best shown in FIG. 2, the exteriorthreads 42 are interengaged within the interior threads 34 so that eachincludes a portion of its length in a meshing relation which isgenerally equal to the total length of the threads provided.

The thermoplastic locking compound engages the entire thread surfacearea of the interior and exterior threads and fills the space betweenthe thread surface areas. In accordance with the principles of thepresent invention, applicants have found that, when sufficient threadsurface area has been provided and the clearance space therebetween iswithin certain limits the resultant connection can be adjusted byundertaking the steps of heating the connection to a temperaturesufficient to soften the thermoplastic locking compound as, for example,to a temperature of approximately 350° when using the preferred compoundnoted above, effecting a relative turning movement between the piston 18and the fastener driving element 22 utilizing flats 38 and 44 while thethermoplastic locking compound is in a softened condition to an extentsufficient to establish a desired position of adjusted operation.Finally, the thermoplastic locking compound is allowed to harden.

The extent of the turning action will be dependent upon the purpose forwhich the adjustment is made. In some instances, the purpose of theadjustment is to extend the fastener driving element into the piston sothat the fasteners being driven will be driven flush rather thancountersunk. An adjustment into this position would require a turningmovement of the fastener driving element within the piston in acounterclockwise direction as viewed from the top. A more typicalpurpose for effecting movement of the piston and fastener drivingelement into an adjusted position of operation is that the device hasbeen used with the piston driver assembly in its initial operatingposition for a period of operation the duration of which is sufficientto reduce the extended dimension of the fastener driving element. A wearextent of one-eighth of an inch is a typical amount which wouldnecessitate readjustment, although one-sixteenth of an inch may beenough in some instances depending upon the particular use. To effect anadjustment for this purpose, a clockwise turning movement of thefastener driving element with respect to the bore is made while thethermoplastic locking compound is in a softened condition. As many asthree or more such adjustments can be made in accordance with theprinciples of the present invention. At the end of the adjustment rangeprovided, the piston 18 and fastener driving element 22 assume a finalposition of operation, such as shown in FIG. 3. In this position, itwill be noted that the portions of the lengths of the threads which arein intermeshing relation are less than the portions of the lengths ofthe threads which are in meshing relation in the initial portion ofoperation shown in FIG. 2. It will be noted that the initial portions ofthe lengths of the threads which are in meshing relation have portionswhich are common to the final portions of the length of the threads andthese common portions are equal to the final portions.

The effectiveness of the thermoplastic locking compound to secure thepiston and fastener driving element together is a function of the amountof thread surface area contacted by the thermoplastic locking compoundand the thickness of the compound extending between the thread surfaceareas contacted. The thickness of the compound between the threadsurface areas is a direct function of the clearance space which, inturn, is a direct function of the difference between the pitch diameterof the interior threads 34 and the pitch diameter of the exteriorthreads 42.

In accordance with the principles of the present invention, this pitchdiameter difference should be within an operative range of between0.000" to 0.013". It is preferably within a range of 0.000" to 0.009".This difference in the pitch diameters of the threads is also a directfunction of the class fit between the threads. A preferred exemplaryembodiment is to utilize an American standard unified class 3B threadfor the interior threads 34 or an equivalent thereof in another profileand to utilize an American standard unified class 3A thread for theexterior threads 42 or an equivalent thereof in another profile.

With respect to the amount of thread surface area which must beprovided, this will depend to a large extent upon the power level of theparticular piston-driver assembly. This power level is a function of theoperative surface area of the piston. Moreover, not all of the threadsurface area provided enters into the effectiveness of the connection.Indeed, it is only the common portions which are in meshing relation inany position of operation. In addition to the effective length ofthread, the thread surface area is also a function of the thread pitchand the pitch diameter. A preferred pitch in the exemplary embodimentpreviously given is a UNF or fine pitch thread, although a UNEF or extrafine pitch thread may be utilized or a UNC or coarse pitch thread mayalso be utilized if desired. An expression of the required threadsurface area which takes into account the variation for power size andvariation in thread lengths in meshing relation is the ratio of thetotal thread surface area provided by the common lengths of both theinterior threads 34 and the exterior threads 42 such as shown in FIG. 3to the operative piston area provided by the top surface 32. This ratiois operative within a range of 0.700:1 to 0.373:1 and preferably withina range of 0.540:1 to 0.373:1. With the preferred embodiment shown, theratio is 0.373:1.

For practical purposes, it is preferable to keep the pitch diameter ofthe exterior threads 42 relatively small so that the bar stock which isused to form the fastener driving element 22 can be relatively small. Apreferred pitch diameter to length relationship with respect to theportions of the lengths of threads in meshing relation in the initialposition of operation is a ratio of initial meshing thread length topitch diameter of at least 2.2 to 1.

FIG. 5 graphically illustrates the relationship between piston size andtotal intermeshed thread area. The factors which enter into the totalintermeshed thread area are schematically illustrated in FIG. 4. Theseinclude the perpendicular distance between opposed thread surfaces (GAP)and the functional length of one side of opposed intermeshed threads(FL). These factors are in turn a function of the external major threadradius (r) and the internal minor thread radius (r₁). In FIG. 4, r isshown as being equal to the external major diameter plus 0.125 pitchdiameter clearance divided by 2. The radius r₁ is equal to the internalminor diameter minus 0.125 pitch diameter clearance divided by 2. FL andGAP are expressed as follows: ##EQU1##

With reference to the graph of FIG. 5:

    A total=4nπ(1.1547 r.sup.2 -1.1547 r.sub.1 r+r.sub.1 FL)

which simplified gives:

    A total=n(14.51 r.sup.2 -14.51 r.sub.1 r+12.566 r.sub.1 FL)

in which the total number of intermeshing thread surfaces is four, n isthe total number of intermeshing threads and the reciprocal of cosine30° is 1.1547.

The graph of FIG. 5 relates to a preferred embodiment and shows theinitial number of intermeshed 7/16-20 threads when the ratio of totalintermeshed thread area to piston area is 0.540:1. Included in the graphare three thread sizes, located at the logical places to change size.The preferred 7/16-20 size is for pistons from 2" to 23/4" diameterwhich is exemplary of most tools of a full line. For convenience offield servicing of a full line of tools, a "20" pitch would be used forpistons below 2" diameter and above 23/4" diameter.

It thus will be seen that the objects of this invention have been fullyand effectively accomplished. It will be realized, however, that theforegoing preferred specific embodiments have been shown and describedfor the purpose of illustrating the functional and structural principlesof this invention and are subject to change without departure from suchprinciples. Therefore, this invention includes all modificationsencompassed within the spirit and scope of the following claims.

What is claimed is:
 1. A method of adjusting the operating position of afastener driving element of an assembled fastener driving device withrespect to a piston of said device in the axial direction of the pistonwherein connection between the fastener driving element and pistonconstitutes interengaged threads and a thermoplastic locking compoundcoating interengaging thread surfaces and filling spaces therebetween,said adjusting being effected without the necessity of disconnecting theinterengaged threads and of applying new locking compound when athreaded reconnection is reestablished, said method comprising the stepsofheating the threaded connection sufficient to soften the thermoplasticlocking compound, effecting a relative turning movement between saidfastener driving element and said piston while said thermoplasticlocking compound is softened to an extent sufficient to bring saidfastener driving element and said piston into the desired position ofadjustment such that the interengagement of the threads is devoid ofsignificant opposing axial forces acting between the threads, andallowing the softened thermoplastic locking compound to harden.
 2. Amethod as defined in claim 1, wherein said heating step raises thetemperature of said thermoplastic locking compound to approximately 350°F.